MILLIONS of baby girls are believed to have been quietly put to death in China as a result of its rigid 'one-child' policy of population control. In male- dominated rural areas, the pressure to have a son is so strong that girls often disappear shortly after birth, giving parents another chance to ensure their only child is a boy.

Now peasant tradition has found a modern reinforcement - pre-natal scanning. In one rural township where parents have been able to discover the sex of their children in the womb, more than three boys are being born for every two girls. The Communist Party committee in Zaozhuang, in the northern province of Shandong, has warned that officials who have used scanning to detect and abort female babies will be stripped of their posts and membership.

Obtaining evidence of this secret slaughter has never been easy, except for the growing disparity in population statistics. Normally 105 or 106 girls are born for every 100 boys, but the past three censuses in China have recorded more than 110 boys aged 12 months or less for every 100 girls. According to the semi-official China News Service, the gap is far wider in rural areas: newborn boys outnumber girls by an average 144.6 to 100. In Zaozhuang the ratio is 163.8 to 100, an imbalance the agency says is being blamed on pre-natal scanning.

The problem is by no means unique to China. Last week Punjab, where there are only 820 women to every 1,000 men, became the second Indian state after Maharashtra to ban pre- natal sex determination tests, following large-scale abortions of female foetuses. Doctors who abort girls after a test face up to three years in jail under the new law. Britain has no compulsory national guidelines, although the General Medical Council warns that it is unethical to perform abortions on sex grounds except in the case of genetic disorders which are passed on to the next generation only by one sex.

In China, the combined effects of two vast exercises in social engineering are proving lethal to girls. The one-child policy is brutal enough - transgression of it can result in houses being pulled down, peasants being fined a year's wages and heavily pregnant women dragged from their homes at night to be bullied into immediate abortions. Human rights groups say it is also being used to prevent ethnic minorities, who previously had fallen outside its net, becoming a larger proportion of the population.

In Tibet, it is feared that ruthless birth control and the influx of Chinese settlers are making Tibetans a minority within their own borders. But China's rulers are trying to improve the quality of the population as well as limiting the quantity. Theories of eugenics from the 1930s and 1940s, discredited elsewhere, are current: 'Apart from failing to understand the general moral implications of eugenics policies, they seem unaware that scientifically they don't work,' said Frank Dikotter of the School of Oriental and African Studies in London. 'You can't improve quality by eliminating so-called 'bad strains'.'

The Prime Minister, Li Peng, showed the official understanding of such matters, however, when he said in 1990: 'Mentally retarded people will give birth to idiotic children.'

Peking last year put forward a draft law on Eugenics and Health Protection 'to avoid new births of inferior quality and heighten the standards of the whole population'. It proposed that those with hepatitis, venereal disease or mental illness should be banned from marrying, pregnant women with 'certain infectious diseases' should be advised to halt the pregnancy, and couples with diseases 'should have themselves sterilised'. The authorities were unapologetic when the bill provoked an international outcry.

Sheila Hillier, Professor of Medical Sociology at London Hospital Medical College, said China had an extensive programme of genetic counselling before marriage, aimed at eliminating inherited disabilities. 'Unfortunately,' she added, 'many people seem to consider being a girl a genetic defect.'

There are already frequent reports of men in parts of rural China complaining of the shortage of marriageable women. One result is the growing incidence of women being kidnapped and sold as brides, an offence for which traffickers have been executed. 'It is an economic problem,' Professor Hillier said. 'Poorer men have trouble finding wives, while those with money can afford more than one.'

But if birth control and eugenics continue to do their work, it is not simply going to be poor peasants who face a lifetime of celibacy. Some projections claim that the Chinese population, which the authorities admit will probably reach 1.2 billion this year and will continue growing at least until 2020, could plunge thereafter to 700 million. It is far more likely, however, that social attitudes will change well before then, even if there is little sign of that happening so far.

'If China is hoping that rising prosperity will encourage people to have fewer children, it had better think again,' said Professor Hillier. 'Demographers are beginning to revise their ideas. It now appears that both above and below a certain band of incomes, people feel they can afford a bigger family.

'Emancipating women, especially raising their educational level, is by far the strongest factor in limiting the number of births. In China it would have the double effect of making girls more valued and stabilising the reproductive capacity of the population as a whole. But it doesn't seem to be grasped as a major policy issue.'

The elimination of baby girls in China, said Gerald Segal of the International Institute of Social Studies, 'challenges the assumption that new technology and greater prosperity always make things better. It depends on all sorts of other factors, such as social attitudes and cultural bias. In China it is making the problem worse, because they are half a generation away from a peasant society.'